1. Field of the Invention
The present invention relates to vehicular load-leveling devices, and more particularly to a level sensor used in conjunction with such load-leveling devices to maintain proper vehicle attitude.
2. Description of the Prior Art
Load-leveling devices are used to maintain the proper attitude of a vehicle subjected to various loading conditions. Improper vehicle attitude can affect many important operating parameters, including, for example, bumper height, rear vision, forward lighting and overall ride quality. The conventional measure of vehicle attitude is the axle-to-body distance, i.e. the distance from the axle to a fixed reference point on the frame. This measure must be kept within a permissible range of values to assure proper vehicle operation.
An air-spring shock absorber is a vehicle load leveling device that can be adjusted to accommodate various load conditions. The shock absorber is generally mounted with one end fixed to the axle and the other end fixed to the vehicle frame. It comprises a closed tubular member carrying a piston-cylinder arrangement for the damping out of road distrubances. The outer end of the closed tubular member contains an air volume which is pressurized by an air pump. A change in air pressure will cause a corresponding change in air volume, which in turn can be translated into a change in axle-to-body distance.
To maintain the axle-to-body distance within a permissible range of values, a transducer is needed to monitor the positional relationship of a reference point on the frame with respect to the axle. One means of accomplishing this is to sense the variations in the relative position of a point on the piston-cylinder arrangement along the axial dimension of the closed tubular member.
However, there are basic design problems associated with providing a position sensor within the volume defining the air spring. Most fundamental, is the need to maintain the integrity of the pneumatic seal within the closed tubular member to preserve the pressurization. A second problem is the need for a sensor of high durability to withstand the vibration and disturbances experienced within the environment of the shock absorber. A third consideration is the need for the sensor to be readily adaptable to the various types of production shock absorbers. Moreover, the sensor must be reasonably easy to manufacture and low in cost to be commerically feasible.
The patent to Wallace, U.S. Pat. No. 3,269,685 discloses a level sensor for use in conjunction with a hydraulic load-leveling device. The sensor comprises a pair of mutually perpendicular tracks or grooves in the wall of the load-leveling device. Each track or groove has a ball disposed for free motion between two extreme positions. The position of the ball is responsive to the orientation or position of the associated track. If the ball reaches too extreme a position away from the center of the track it will interrupt a beam-of-light (BOL) sensor providing a signal indicative of the misorientation or mispositioning of the load-leveling device.
The relatively complex mechanical character of the Wallace sensor limits its suitability for use within a load-leveling shock absorber. More specifically, it does not appear to have the needed durability or reliability. In addition, it would be costly to manufacture and difficult to equip on a production model shock absorber.
The patent to kirschner et al, U.S. Pat. No. 3,871,682 discloses another sensor for use in conjunction with a load-leveling device. The Kirschner et al sensor employs a light source and first and second photoresistors. The first and second photoresistors define the upper and lower bounds of permissible axle-to-body distance. However, the Kirschner sensor has a light source and photoresistor which are mounted on the vehicle body and a light filtering element is mounted by an arm on the axle to move between the source and the photoresistor
The patent to Jackson, U.S. Pat. No. 3,606,375, discloses a mechanical level-sensor for an air-spring shock absorber. The Jackson sensor employs a pivotal sensor arm that monitors the position of a relatively movable member. The sensor arm actuates a pair of switches through a camming arrangement. The Jackson sensor employs complex mechanical structure and requires extensive modification of the tubular member defining the air-spring.
Other prior art devices, including those taught in Hagwood, U.S. Pat. No. 3,525,512; Wanner, U.S. Pat. No. 3,666,287; Jackson, U.S. Pat. No. 3,372,919; and Elliott et al, U.S. Pat. No. 3,575,442, are similarly attended by the same disadvantages of cost and complexity as those sensor devices previously discussed
It is a principal objective of the present invention to provide a level sensor for use in conjunction with a load-leveling shock absorber that overcomes the limitations of the prior art. More specifically, the level sensor should be functional in a pneumatic or hydraulic device, have good durability and reliability, and be reasonably inexpensive to manufacture.